Modern digital systems are commonly simulated in software, in order to assure proper functioning, prior to actual fabrication in hardware. However, as digital systems become more complex, the simulation itself can also become quite complex.
The increased complexity can cause several phenomena. One is that the number of states within the system can become so large that even a simulation cannot test all possible states. That is, the system response to all possible inputs cannot be checked. A second is that, even if a reduced set of inputs is used, the reduced set can produce so much data that analysis by humans becomes impractical.